One of the seminal developments in the industrial minerals field was the discovery that calcination of a particular type of poorly crystallized, ultrafine tertiary kaolin clay, known in the art as "hard" kaolin, resulted in bright, e.g., 93.5% GE brightness, pigments having desirable low abrasion and outstanding potential to provide opacity to filled and coated paper products. Reference is made to U.S. Pat. No. 3,586,523, Fanselow et al (1971). Products made in accordance with the teachings of this patent by the assignee and its licensees have accounted for a substantial proportion of the sales of high performance industrial minerals in the United States and abroad. Aside from possessing a unique combination of high brightness, low abrasion and opacification potential, the processing was remarkably economical because of the unique morphology as well as inherently fine particle size and desirable particle size distribution of the kaolin in the crude. In such crudes, colored impurities, in particular iron-contaminated titania particles, concentrate in the coarse fraction of the crude. Thus, conventional degritting and fractionation to recover calciner feed yield a fine fraction of hydrous kaolin with a lower content of colored titania than the crude. As a result of these factors, the yield of calcined kaolin product was high, e.g., roughly about 50% based on the weight of degritted crude or about 40% by weight of the particles finer than 1 micron in the degritted crude.
Unlike the hard Georgia Tertiary kaolins that are poorly crystallized and have an average particle size of about 0.3 microns, (equivalent spherical diameter or e.s.d.), the soft kaolins that are present in even greater abundance in Georgia, U.S.A. and throughout the world are composed of much larger, well crystallized kaolin particles having an average particle size of approximately 0.6 microns, e.s.d., roughly twice that of the hard kaolins. Fractionation of a soft clay to isolate the fine size fraction of the crude having generally the same average particle size and particle size distribution of a typical degritted hard kaolin would represent only about a 15% yield (or 85% loss) of degritted kaolin. Thus, assuming that calcination of such a fine fraction of a degritted soft kaolin would result in a bright, low abrasion opacifying pigment, at least comparable in all significant performance characteristics to calcined pigments prepared from hard clay, the processing would not be economically viable.
It is well known in the industry that the brightness of uncalcined and calcined kaolin pigments is adversely affected by the presence of colored impurities, notably titaniferous and ferruginous impurities. It has long been the practice to remove these impurities to various extents by physical or physical-chemical means, such as froth flotation, selective flocculation, magnetic purification, bleaching and combinations thereof. It is also known that calcination generally increases brightness of a hydrous kaolin when the clay is "fully" calcined, i.e., calcined to undergo the characteristic exotherm, and may decrease in brightness when the clay is calcined under less severe conditions, i.e., to so-called "metakaolin" state. Thus, 90% brightness (fully) calcined pigments can be produced readily from hard crudes by degritting, fractionation, bleaching and calcination; 93% brightness pigments can be obtained from the same crudes by adding one or more steps to remove colored impurities, especially titaniferous matter. The Fanselow et al patent (supra) discloses 95% brightness calcined kaolin pigments.
Attempts were recently made to produce calcined pigments having much higher brightness from the hard kaolin crudes that are readily processed to provide 93+% brightness calcined kaolin pigments. To the best of our knowledge, there is at present no means to produce calcined kaolin pigments having a 95+% brightness merely by decreasing the content of colored impurities in the hard kaolin by conventional means such as flotation.
On the other hand, there are reports in the patent literature dating back to the early '60's of the recovery of calcined kaolin pigments having 96% brightness and higher from certain Georgia kaolins crudes. These crudes are obviously of the soft type as evidenced by information about particle size and particle size distribution in the patents. Reference is made to the following:
U.S. Pat. No. 3,058,671 (1962) Billue PA1 U.S. Pat. No. 3,343,973 (1967) Billue PA1 U.S. Pat. No. 3,171,718 (1965) Gunn, et al PA1 a) providing a degritted soft kaolin crude that is from 50-65% by weight finer than 2 microns, e.s.d. and contains substantially all the minus 1 micron particles present in the degritted naturally occurring clay, the degritted crude containing from 0 to 1.8% by weight TiO.sub.2 and 0 to 0.8% by weight Fe.sub.2 O.sub.3 based on the dry weight the degritted crude, and having low levels of mica, quartz or other silica minerals and montmorillonite minerals; PA1 b) removing colored impurities by selective flocculation, froth flotation or combinations thereof to reduce the titania content thereof to a level below 1% if the titania content exceeds 1% in step (a); PA1 c) agitating the degritted kaolin clay in the presence of water with particulate grinding media until the particle size is from 85-92% by weight finer than 2 microns; PA1 d) removing sufficient particles larger than 2 micron from the product of step (c) by sedimentation or centrifugation to produce a product that is about 95-100% by weight finer than 2 micron and about 88-92% finer than 1 micron and to provide calcined feed that is at least 40% by weight of the clay from step (a); preferably bleaching the fractionated clay; PA1 e) spray drying the recovered product of step (d) pulverizing, fully calcining and repulverizing in conventional manner and; PA1 f) recovering the calcined product having a GE brightness of at least 95%, and Einlehner abrasion below 25 and a white color. (L value greater than 98) PA1 a) selecting a soft kaolin clay crude that contains less than a total of 1% by weight quartz or other crystalline silica minerals, mica, and smectite minerals in the minus 2 micron fraction thereof, the crude containing less than 0.8% Fe.sub.2 O.sub.3 and less than 1.8% TiO.sub.2 after removal of grit in step (b), PA1 b) removing grit from said crude, PA1 c) fractionating the resulting, degritted crude and recovering a fine size fraction thereof that is from 60 to 85% by weight, preferably about 76% to 82%, e.g., about 80% by weight, finer than 2 microns, e.s.d., PA1 d) subjecting the resulting fraction from step (c) to flotation, selective flocculation or combination thereof to remove colored impurities and recovering a beneficiated fraction of clay that contains from 0-0.8% TiO.sub.2 and from 0-0.8% Fe.sub.2 O.sub.3, PA1 e) agitating the beneficiated fraction of kaolin with particulate grinding media until from 95-100% by weight is finer than 2 microns, e.s.d., PA1 f) removing sufficient particles larger than 2 micron e.s.d. from the product of step (e) to result in calciner feed that is about 90-95% finer than 1 micron, e.s.d., and represents at least 50% by weight of the degritted crude from step (b); optionally bleaching, PA1 g) spray drying the recovered product of step (f); PA1 h) pulverizing, fully calcining and repulverizing the product of step (g) in conventional manner and; PA1 i) recovering the calcined product having a GE brightness of at least 95%, an Einlehner abrasion below 25 and a white color--(L value of at least 98).
Billue allegedly discovered that media grinding of the coarse fraction of kaolins resulted in "fracture-induced" brightness and found that the enhanced brightness was carried over when the fractured kaolin was fully calcined. Specifically, Billue isolated from a naturally occurring crude a coarse fraction that contained not more than 35% by weight of particles finer than 2 microns, i.e., a coarse fraction of crude containing at least 65% by weight of particles larger than 2 microns. In Example II (both patents), a coarse fraction of kaolin was "fractured" to produce a fractured clay having an initial brightness of 89.1%. This hydrous kaolin was calcined to produce a product having a brightness of 96.3% (col.7). However, data in the patent reveal that the portion of the clay that was fractured (the 86% minus 2 micron cut) represented a meager 28.8% by weight of the crude. Thus, while calcined brightness was very high, the process depended upon the removal of minus 2 micron fines before grinding and would be commercially useless unless the higher brightness would compensate for the high cost associated with low yield. Billue did not report the abrasion value or provide information about the opacification potential of his 96.3% brightness calcined clay. Gunn et al teach media grinding of coarse filler clay in the production of calcined kaolin pigments made by mechanical delamination of a coarse hydrous filler clay fraction. Disclosed is a product having a GE brightness value of 95% and "low" abrasion as obtained by a specific modification of a Valley Abrasion test. The clay was delaminated gently to separate kaolin booklets (as contrasted with severe grinding to break particles after delaminating the clay). In an example, delaminated kaolin having a GE brightness of 92.1% was calcined to a GE brightness of about 96%. Gunn et al do not report data such as to permit calculation of yield, however, the teaching of Billue (supra) provide guidance and suggest yield would be low.
The Gunn et al patent also clearly teaches that the finest fraction of the kaolin has the highest content of TiO.sub.2 impurity (Example 9). In other words, in this type of kaolin, colored impurities concentrate in the fine end. This is well known to clay mineralogist and the clay industry. It is noted that Gunn et al did not disclose calcining the fine fraction. However, based on the disclosure as to the concentration of colored impurities in the fine end of soft kaolins, one would avoid the presence of excessive fines if seeking brightness from a purified kaolin obtained from a soft kaolin crude. This is consistent with the specific disclosure in the Gunn et al patent of ultrahigh brightness calcined pigments made by processing the coarse fraction of a crude after the removal of fines.
In U.S. Pat. No. 5,137,574, Suitch et al, a high opacifying pigment is produced by calcining one or more high titania content kaolin fractions which are separated from a whole crude by size classification, froth flotation, magnetic separation or the like. In embodiments of the invention, a grinding step is practiced before calcination. The grinding step is said to be necessary in order to reduce the particle size by virtue of the use of a titania enriched product. The calcined product has a brightness in the range of about 70 to 82% and the titania content in greater than 2%. The low brightness product is supplied under the registered trademark OPACITEX. Production of such a pigment by selection of a low brightness kaolin crude is described in U.S. Pat. No. 5,371,051, Pope et al.
McConnell et al, U.S. Pat. No. 4,381,948, now commonly assigned, discloses a process for producing high brightness, low abrasion calcined kaolin pigments having exceptionally high light scattering (superior opacification), a GE brightness of at least 93% and low abrasion (Valley Test Method) when used as a paper filler. Processing involves use of a crude clay containing not more than 0.5% glass forming oxides, not more than 1.5% Fe.sub.2 O.sub.3 and more than 2% TiO.sub.2 and separating a very fine particle size fraction from such crude, i.e., a fraction that is 100% by weight finer than 1 micron, followed by conventional steps of drying, pulverizing, calcining and repulverizing. In all illustrative examples, a hard kaolin crude was processed. The highest brightness disclosed was 94.3%. The patent also teaches that soft Georgia kaolin crudes (identified as crudes that are 50-60% finer than 2 microns) can be used. However, the inventors caution that in such case the particular crude should contain sufficient 1 micron particles to enable "worthwhile recoveries". Teachings sufficient to suggest, much less to enable, the achievement of worthwhile recoveries are not presented in making the disclosure as to the usefulness of soft kaolins.
Moreover, patentees did not deal with the problem inherent in the use of a very fine fraction when using soft kaolin because of the concentration of colored impurities, especially colored titania, in the finest fraction of soft kaolins.
An early patent in the art, U.S. Pat. No. 3,014,836, Proctor, discloses that the particle size of a calcined kaolin product depends on the particle size of the uncalcined kaolin feed and that the feed should be free from abrasive impurities to produce low abrasion, calcined clay. The highest GE brightness value was 93%.
U.S. Pat. No. 3,519,453, Morris et al, mechanically delaminate booklets of a coarse kaolin and calcine to metakaolin conditions in an effort to obtain low abrasion; GE brightness was below 90%. Slepetys et al, U.S. Pat. No. 5,393,340, commonly assigned, severely grind booklets in a coarse kaolin fraction so as to obtain a ground material, the particles of which are substantially all finer than 1 micron as a result of grinding. Patentees then calcine to metakaolin to obtain ultralow Einlehner abrasion; brightness was about 90%. Thus, ultrahigh brightness calcined kaolin, 95-96% and above, has been reported as has calcined kaolins characterized by low abrasion (by various methods including the now obsolete Valley method), high opacification or combinations thereof. Prior to this invention, however, brightness values above 95% have been achieved only by processing fractionated soft kaolin crude by steps that resulted in economically unattractive yields and do not inherently provide calcined kaolin pigments that also possess sufficiently high light scatter for paper use combined with the extremely low abrasion values now demanded by the paper industry. On the other hand, high yields have obtained when using hard crudes but 96% brightness products were not produced. Grinding and/or delamination prior to calcination have been advocated with kaolin but not in a process that results in pigments brighter than 95% at high yields, for example a recovery in excess of 45%, based on the weight of particles finer than 1 micron in the crude.